The method of extraction of ceramic nuclear fuel package heat-generating element

 

(57) Abstract:

The method includes melting in a vacuum or inert medium metal structural materials and the separation of the melt from the solid fuel. In the melting pot download the bundle of fuel elements in the composition of the fuel assemblies. The melt contains zirconium, iron, chromium, Nickel. Additionally load a portion of the fragments of zirconium membranes. Additionally load the portion of spent fuel elements without end fittings. The content in the melt, zirconium, iron, chromium and Nickel supported at the level of 72-86; 2-26; 0.2 to 7.0 and 0.1 to 4.0 wt.% respectively. In the melt add zinc in an amount of 0.1 to 15.0 wt.%. The process is conducted at 800-1250oC. the Method reduces the complexity of the process of separation of ceramic nuclear fuel from the metal structures of the fuel elements and to reduce the number of generated radioactive metal waste. 11 C.p. f-crystals, 1 Il.

The invention relates to a pyrometallurgical methods of reprocessing spent nuclear fuel (SNF) mainly on the basis of uranium.

There are ways for mechanical recovery of uranium oxide fuel from the metal is of iesa:

in the disassembly of the fuel assemblies (FA) to separate the fuel and the subsequent cutting [1], cutting [2] or strain [3];

- cutting tool and cutting all fuel assemblies into separate pieces without disassembly into individual fuel rods [4].

The disadvantage of these methods is the formation of large quantities of dust and aerosols oxide fuel, which requires a powerful and highly efficient gas cleaning systems. In the cutting of fuel rods is formed of a large number of fragments of zirconium membranes, the so-called zirconium husks, which are flammable and require the construction of special stores large amounts.

The closest in technical essence and the achieved result is a thermal method of extraction of ceramic uranium fuel package heat-generating element [A. S. USSR 357596, publ. 31.10.1972, bull. N 33] separating fuel from structural materials by heating to the melting temperature of the membranes, according to which the fuel elements are placed in a container made of iron 15-25% by weight of structural parts package fuel elements and performing the role of solid additives, and lead the process of opening in vacuum or inert atmospheres the sun by cutting shank and retrieve packages rods, that is a very tedious operation performed using manipulators, because you are working with irradiated fuel assemblies;

- the need to produce containers of iron, which during the processing of the fuel rods will be part of the radioactive metal waste and cannot be extracted economically acceptable way.

The technical result of the proposed solutions is to reduce the complexity of the process of separation of ceramic nuclear fuel from the metal structures of the fuel elements and reducing the number of forming a radioactive metal waste. The technical result is achieved in that in the method of extraction of ceramic nuclear fuel package heat-generating element, comprising melting in a vacuum or inert medium metal structural materials and the separation of the melt from the solid fuel according to the invention melt is subjected packages of fuel elements in the composition of the fuel assemblies in the melting crucible with the melt containing zirconium, iron, chromium and Nickel.

In the crucible with the melt simultaneously emitting Assembly additionally load a portion of the fragments of zirconium is niteline load portion of the spent fuel elements without end fittings.

The content in the melt, zirconium, iron, chromium and Nickel supported at the level of 72-86; 2-26; 0.2 to 7.0 and 0.1 to 4.0 wt.% respectively.

In the melt add zinc in an amount of 0.1 to 15.0 wt.%.

The process is conducted at 800-1250oC.

Part of the melt after separation from the solid fuel is directed to the extraction operation ceramic nuclear fuel package heat-generating element.

The melt after separation from the solid fuel filter.

Solid fuel washed zinc melt.

Leaching zinc melt after separation from the solid fuel filter.

The process is conducted in a vacuum induction furnace with copper split water-cooled crucibles with electromagnetic stirring of the melt.

The process is conducted at a frequency induction currents 50-250000 Hz.

The main distinctive features of the proposed method is that the processing is subjected to the fuel assemblies in the configuration in which they were extracted from a nuclear reactor, without any prior mechanical disassembly.

The process is carried out by immersing the fuel assemblies in the melting crucible with the melt on the basis of zirconium, the soda is Menno: cladding and grid spacers, made of Zirconia doped with niobium, and end parts (shank), made of steel 12X18H10T. This steel contains Fe, Cr and Ni, which form with zirconium fusible eutectic alloy. As a result of dissolution of the metallic component of the fuel assemblies will be formed two-phase system consisting of molten metal and solid uranium dioxide, with subsequent separation of the phases by density directly in the melting crucible and a control filtering molten metal, with the aim of separating suspended particles of fuel from the discharge him from the crucible.

Diagram of the fuel assemblies of VVER-1000 reactor shown in the drawing. It consists of:

loading and unloading 1 and boarding 4 heads, so-called end pieces or shank made of stainless steel 12X18H10T, through which the fuel rods are rigidly fixed in a certain position;

package heat-generating element 2, which is a set of tubes of zirconium doped with niobium, which is located inside the oxide uranium fuel;

- the spacer hex cell arrays 3 of Zirconia doped with niobium, through which pass the rods.

About the girovanny niobium - 184

Stainless steel 12X18H10T - 106

The calculation shows that melting metal component FA of the resulting multicomponent Zr-Fe-Cr-Ni melt will have a mass of 290 kg and contain, wt.%:

- stainless steel components - 36,3, including Fe - 26, Su - 6.6 and Ni - 3,7, respectively;

- Zirconia doped with niobium (by difference) - 63,7.

It is found experimentally that for a given multicomponent system minimum liquidus temperature of 800-1200oTo have the alloys containing in total 14-28 wt. % the components of stainless steel 12X18H10T: Fe, Cr and Ni. The process at these temperatures minimizes the probability of recovery of uranium dioxide-zirconium to uranium metal, which, in turn, dramatically reduces the loss of uranium zirconium melt.

To obtain an alloy of the above composition should be increased content of zirconium with 63,7 to 72-86 wt.%. This is achieved by loading in the melting pot along with regular TVs up to 200 kg of fragments of irradiated zirconium membranes, the so-called zirconium husk, accumulated in large quantities in the result of the reprocessing of irradiated fuel elements by the method of felling for several decades. The zirconium content in rasatala.

If necessary to further reduce the liquidus temperature (up to 750oC and below) in the melt, it is advisable to add 0.1 to 15.0% zinc, forming a zirconium and iron fusible alloys. This will further reduce the degree of reaction recovery of uranium to the metallic state.

The content of iron, chromium and Nickel in the melt, is equal to: 2-26, 0,2-7 and 0.1 to 4.0 wt. % respectively, is determined mainly by the content of these metals in the stainless steel 12X18H10T, which are made of the end parts assemblies, and the number added to the melt zirconium bearing materials - zirconium husk or rods without end fittings. If necessary, adjustment of the composition in the melt to impose an additional small amounts of iron, chromium and Nickel.

To remove residual metallic zirconium-iron-chromium-Nickel melt from the pores of the solid oxide uranium fuel (fuel oil) is washed with a low-melting zinc melt. Zinc melt, it is advisable to use repeatedly, periodically cleaning (regenerating) from accumulating in the leaching process of zirconium, iron, chromium, Nickel and other impurities.

For technically and economically effective about the progressive environment of molten metal required special melting furnace, to implement the following conditions:

to exclude contamination extractable nuclear fuel material in the melting crucible;

- exclude the absorption of a melting crucible of radioactive elements to avoid the formation of a new type of radioactive waste in the form of waste crucible materials;

to ensure electromagnetic stirring of the melt, stimulating the dissolution of structural materials of Fe in the melt on the basis of zirconium;

- ensure that the technological operations in the desired atmosphere: vacuum, inert gas, air, etc. and should be provided with the ability to change the atmosphere in the process;

to ensure that the movement of the melt inside the crucible in the desired direction (up or down), and to merge or to crystallize the melt and extrude in the form of a compact ingot;

to provide a radical solution to the problem of durability of the crucible, namely: it should work for several years and do not require intermediate sweeps and repairs involving staff.

These properties are vacuum induction furnace with metal split water-cooled (cold)polozhennymi in the upper part of the crucible. As demonstrated industry experience, life IPHT reaches 18 years of age and over, which virtually eliminates the formation of such a class of radioactive waste, spent as crucible materials. The use of induction currents of relatively low frequency range (50-250000 Hz) allows the use of simple and reliable sources of power supply, and also greatly simplifies the design IPT.

Before beginning the process of processing in a cold crucible equipped with a drain trough in the upper zone and moving inside a water-cooled copper tray with drain device, preparing the bath fusible melt on the basis of zirconium containing Fe, Cr and Ni, the proposed composition of which is indicated above. Spent fuel assemblies and a portion of zirconium husk or spent fuel rods evenly immersed in the melt, where they dissolve under the influence of such factors as high temperatures, corrosive chemical environment of the metal melt and his intense electromagnetic stirring. It is obvious that the physico-chemical characteristics of the metal parts of the fuel assemblies after dissolution will be concentrated in the metal melt. In a cold crucible as the processing th is its oxide fuel of various sizes. The latter, as having a greater density (10-11 g/cm3will settle to the bottom of the crucible. The metal melt containing small amounts of fine particles of oxide fuel in suspension, will be located in the upper part of the crucible. The metal melt will fill the open pores of the oxide fuel.

After processing a batch of fuel assemblies, when the quantity of solid fuel in the crucible reaches the maximum allowable filling, melt the zirconium is poured in the groove in the mold by moving up the pallet. The melt in the mold is crystallized, cooled ingot is extracted and sent to storage. It should be noted that compact ingots will require significantly less storage, and will be completely fireproof unlike zirconium husk stored currently in bulk.

The metal melt is located in the pores of the solid fuel, remove to a heated mold through the device to the bottom drain. After washing the zinc melt the solid fuel is unloaded from the crucible and sent for further processing. Present in the poured molten solids SNF if necessary, hoteltravel is dnow FA and the estimated portion of zirconium husk or spent fuel rods, i.e., the loop processing of the next batch of FA repeats.

Experimental validation of the proposed method was carried out in an induction furnace with transparent for electromagnetic field copper split water-cooled (cold) crucible with a diameter of 100 mm with a device for bottom discharge of the melt. Under drain device installed ceramic filter. Current frequency power supply 2400 Hz provides intensive mixing of the molten metal.

Example 1. The original charge weight 4268 kg simulating fuel assemblies of VVER-1000 reactor, consisted of eight zirconium tubes filled with sintered pellets of uranium dioxide, as well as pieces of stainless steel 12X18H10T. The mass of uranium dioxide was 2485 g of zirconium - 920 g, stainless steel 12X18H10T - 503,

The mixture of the indicated composition was loaded into a cold crucible with the melt mass 3423 g containing zirconium, iron, chromium and Nickel and held for 27 min at 1050oIn an argon atmosphere until dissolved metal fractions of the mixture in the melt and dividing by the density of the melt and solid uranium dioxide. The total mass of the molten metal in the crucible was 4846 g, which had the following chemical composition, wt.%: Zr - 79,73, Fe-is zlonice, where crystallized in the ingot.

The analysis showed that the uranium content in the resulting ingot weight 4770 g was 0.36 wt.%. Residual total content of the metal phase in the uranium dioxide was 1.9 wt.%.

Example 2. The experiment was carried out as described in example 1. The difference was that in the melt mass 3423 g containing zirconium, iron, chromium and Nickel, were added 250 g of zinc. In the resulting melt mass 3673 g downloaded the charge weight 4268 kg, the composition of which is given in example 1. The process was performed for 19 min at 950oIn an argon atmosphere until dissolved metal fractions of the mixture in the melt and dividing by the density of the melt and solid uranium dioxide. The total mass of the molten metal in the crucible was 5096 g, which had the following chemical composition, wt.%: Zr - 76,12; Fe - 13,79; Cr - 3,25; Ni - 1,93; Zn - 4,91. After that he opened the tube of the discharge device and the melt passing through the filter, fell into the mold, where it crystallized in the ingot.

The analysis showed that the uranium content in the resulting ingot weight 5035 g amounted to 0.23 wt.%. Residual total content of the metal phase in the uranium dioxide amounted to 1.2 wt.%.

Example 3. The experiment was carried out, the AC had reload 3 kg of zinc, melted and kept the melt at 800oC for 10 minutes Then zinc melt was poured through the device for bottom discharge was passed through the filter and were led. The analysis showed that the uranium content in the ingot zinc mass 3040 g amounted to less than 0.02 wt.%. The residual content of the metal phase in the uranium dioxide was 0.6 wt.%.

Examples confirm the effectiveness of the proposed method.

Literature

1. Patent UK 1096745, 1967.

2. Patent UK 1171257, 1969.

3. Abdel-Rassoni A. et al. J. Nuclear Energy, 1969, 23, R. 551.

4. Kondrat'ev A. N. and others in Proc. of the Symposium Proceedings CMEA "research in the field of reprocessing of irradiated fuel, so 1, Prague, ed. The AEC, USSR, 1972, S. 174.

1. The method of extraction of ceramic nuclear fuel package heat-generating element, comprising melting in a vacuum or inert medium metal structural materials and the separation of the melt from the solid fuel, characterized in that the melt is subjected packages of fuel elements in the composition of the fuel assemblies in the melting crucible with the melt containing zirconium, iron, chromium and Nickel.

2. The method according to p. 1, featuring the of mentov zirconium clad spent fuel elements.

3. The method according to p. 1, characterized in that the crucible with the melt simultaneously with the fuel Assembly load additional portion of spent fuel elements without end fittings.

4. The method according to any of paragraphs. 1-3, characterized in that the content in the melt, zirconium, iron, chromium and Nickel supported at the level of 72-86; 2-26; 0.2 to 7.0 and 0.1 to 4.0 wt. % respectively.

5. The method according to p. 1, characterized in that the melt add zinc in an amount of 0.1 to 15.0 wt. %.

6. The method according to p. 1 or 5, characterized in that the melting is carried out at 800-1250oC.

7. The method according to p. 1, characterized in that the part of the melt after separation from the solid fuel is directed to the extraction operation ceramic nuclear fuel package heat-generating element.

8. The method according to p. 1 or 7, characterized in that the melt after separation from the solid fuel filter.

9. The method according to p. 1, characterized in that the solid fuel is washed zinc melt.

10. The method according to p. 9, characterized in that the leaching of the zinc melt after separation from the solid fuel filter.

11. The method according to p. 1 or 9, characterized in that the melting lead in a vacuum induction the>

12. The method according to p. 11, wherein the process is conducted at a frequency induction currents 50-250000 Hz.

 

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